1. Field of the Invention
The present invention relates to a liquid crystal composition having a nematic phase and a positive dielectric anisotropy and a liquid crystal display element comprising this liquid crystal composition. The present invention relates particularly to a liquid crystal display element having a small cell gap and a short response time and a liquid crystal composition having a large optical anisotropy.
The liquid crystal composition of the present invention or the liquid crystal display element of the present invention shall be referred to as the composition or the element respectively. The liquid crystal display element is the general term of a liquid crystal display panel and a liquid crystal display module. The compound represented by Formula (I) is referred to as the compound (I). The compounds represented by the other formulas are shown as well in the same manner.
2. Related Art
In a liquid crystal display element, classification thereof based on an operating mode of liquid crystals includes PC (phase change), TN (twisted nematic), IPS (in-plane switching), STN (super twisted nematic), OCB (optically compensated bend), ECB (electrically controlled birefringence) and VA (vertical alignment). Classification thereof based on a driving mode of an element includes PM (passive matrix) and AM (active matrix) . PM (passive matrix) is classified into static and multiplex, and AM is classified into TFT (thin film transistor) and MIM (metal insulator metal).
These elements contain a liquid crystal composition having suitable characteristics. The following general characteristics are required to this composition in order to improve the general characteristics of the elements.
General characteristicsGeneral characteristicsNo.required for a compositionrequired for an element1.Stable against heatLong life2.Stable against UV ray1)Long life3.Wide range of nematicWide usablephasetemperature range4.Low viscosity2)Short response time5.Low threshold voltageLow power consumption6.Large resistivityHigh voltage holdingratio1)A UV ray can be used in a production process. 2)A composition can be put into a liquid crystal cell in shorter time. 
In a composition used for an AM element, the characteristics of Nos. 1 to 6 are important. In a composition used for a PM element, the characteristics of Nos. 1 to 5 are important. In addition to these characteristics, important as well are characteristics such as an optical anisotropy (Δn) , a dielectric anisotropy (Δ∈) and an elastic constant. Suitable values in the characteristics are varied according to the mode of the element. When a composition is put into a liquid cell having an alignment layer, a liquid crystal molecule stands up by virtue of the action of the alignment layer. A degree of standing up is a pretilt angle. This angle is varied according to the kind of the composition. The large pretilt angle stably aligns the liquid crystal molecule.
In recent years, a technique for producing plates used for an element has been advanced, and a cell gap of the plates tends to be reduced. A cell gap is particularly small in an AM element of a TN mode, an OCB mode and so forth. A cell gap is a distance between two plates and is the same as a thickness of a layer of a liquid crystal composition. In an element of a TN mode, a reduction in the cell gap shortens the response time and is less liable to produce a reverse domain. In an OCB mode, a reduction in a cell gap shortens time (transition time) for transiting from splay alignment to bend alignment and shortens time (response time) required for a liquid crystal molecule to change alignment by a change of an applied voltage.
In these modes, a product (Δn×d) of an optical anisotropy (Δn) and a cell gap (d) is fixed. Accordingly, a composition having a large optical anisotropy is required in order to reduce the cell gap. Compositions having a large optical anisotropy are disclosed in Japanese Patent Application Laid-Open No. 3053/2001, Japanese Patent Application Laid-Open No. 3051/2001, Japanese Patent Application Laid-Open No. 123170/2001 and Japanese Patent Application Laid-Open No. 257535/1992. However, a novel composition having a large optical anisotropy and suitable characteristics is always required in order to improve the characteristics of an element.